Conventional turbine meters of the type used to measure the flow of gas typically operate by converting kinetic energy of the flowing gas to rotation of a turbine that has its axis parallel to the path of gas flow.
These turbine meters typically include an elongated, cylindrical housing that forms a flow path for gas which is flowing within a pipeline in which the housing is mounted. An inlet flow straightener is mounted adjacent to an inlet port in the housing to cause gas flowing from the inlet port to flow in an axial direction within the housing. A measuring rotor is mounted downstream of the inlet flow straightener so as to rotate about a central axis of the cylindrical housing. The measuring rotor has turbine blades installed on it which cause it to rotate in one direction at a speed approximately proportional to the velocity of the gas flowing through the housing.
The theory of operation of turbine meters must include consideration of the fact that the density of gases varies significantly with pressure or temperature. Additionally, because the density of gases is relatively low, consideration must be given to the driving torque from gas required to overcome mechanical friction in a turbine meter. In particular small changes in retarding torques, for example due to increases in friction between moving parts, may affect the performance of turbine meters, especially at low pressure and low flowrates. Changes in kinematic viscosity may also affect the performance of turbine meters.
The total volume of gas passing through the meter is generally determined by counting the number of revolutions of the measuring rotor mounted within the meter. Because of this, turbine meters are also known as inferential meters, since they infer how much gas or liquid has passed through by observing something else i.e. gas velocity. Therefore, the actual flow-rate of a turbine meter can be inferred from the velocity of the gas when the cross-sectional area of the annular passage preceding the rotor is known.
The driving energy to turn the rotor is the kinetic energy, or energy of motion, of the gas being measured. The gas impinges on rotor blades mounted on the measuring rotor and overcomes retarding forces that inhibit the rotor from turning. Because the density of gas is low, it is generally necessary to reduce the cross-sectional area of the gas pipeline in which a turbine meter is mounted to accelerate the flow of the gas to a higher kinetic energy which allows the gas to be measured by the turbine meter. Often, an inlet flow guide, or flow straightener, serves to reduce the area through which the gas flows to approximately one-half the area of the pipe in which the turbine meter is installed. Reducing the cross-sectional area of the flow path of the gas increases the velocity of the gas proportionately when the gas flow-rate remains constant.
Turbine meters are commonly installed in pipelines used in the natural gas industry for the measurement of the flow of large volumes of gas. The volumes that pass through the pipelines are often so large that small errors in measurement can result in large losses of revenue to gas transmission companies and local distribution companies.
For the above reasons, each turbine meter must be calibrated to determine its accuracy after it is manufactured. Calibration is necessary because normal, minor variations in meter components cause each turbine meter to register a slightly different volumetric flow for a given volume of gas. By way of example, from meter-to-meter, blades on turbine measuring rotors vary slightly in shape due to minor manufacturing inconsistencies. As a result, each turbine measuring rotor rotates at a slightly different speed for gas flowing at the same velocity. Similarly, separate sets of measuring rotor bearings of the same make and model can impose slightly different functional forces on the rotors of separate meters on which they are mounted. Additionally, a turbine meter normally has a mechanical register, sometimes called an index, which gives a reading of gas flow volume on a set of dials. A register is typically connected to a turbine-measuring rotor through a coupling, which includes gears, magnetic couplings and other components which load the turbine rotors of different turbine meter to a somewhat different extent. As a result, each turbine meter will register its own unique flow level for a given volume of gas.
Conventionally, at the time of manufacture of a turbine meter, testing the meter against a known standard such as a master meter, a bell prover or a sonic nozzle proves the accuracy of the meter. Testing and calibration is done at a given temperature, a given gas line pressure and a given gas flow rate, which allows the volume of gas registered by the meter to be compared to the actual volume of gas which flowed through the meter as determined by the standard. This ratio of the volume of gas measured by a meter's mechanical register to the actual volume of gas flowing through the meter is called the accuracy of the meter. The calibration factor of a meter, referred to by the letter “K,” is expressed in terms of pulses per unit of volume flowing through a meter and is the amount by which the registered reading of the meter is divided to get a 100% accurate reading. Generally each meter is tested and calibrated based on an air test at atmospheric pressure, the K factors are determined for a range of flow-rates expected for the meter and a table of these K factors may be provided with each meter. A customer may request high-pressure tests, typically at an extra cost.
The accurate testing of a turbine meter after it has been installed is also important because the accuracy of the meter can change over time as a result of factors such as damaged components, increased friction between components due to wear or due to contamination carried by gas flowing through the meter. Thus, there is a need to periodically prove turbine meters over their operating life.
As indicated above, each turbine meter normally has a mechanical mechanism, called a register or an index, which records the volume of gas that has flowed through the meter. The measuring rotor of the meter is coupled through a series of gears, magnetic couplings and the like to a set of dials on the register which indicate the volume of gas that has flowed through the turbine meter. Since only a single set of gears and/or couplings can be installed at one time between the measuring rotor and the dials, the register can only be calibrated to be 100% accurate at one flow-rate, usually about 60% of the maximum flow-rate of the meter.
The accuracy of the volume of gas recorded by the dials of a meters register, however, is checked at the time of a meter's calibration over a range of flow-rates. At any particular line pressure, an accuracy curve is drawn showing the accuracy of the meter as its flow-rate changes. Components of the meter are often modified to attempt to get the accuracy of the meter as consistent as possible over its expected range of flow-rates.
Turbine meters tend to have an undesirable “hump” in their accuracy curve at low flow-rates, signifying that at these flow-rates the register records more gas than has actually flowed through the meter. Generally a meter records less gas than has actually flowed through it below a flow-rate of about 5% of the maximum capacity of the meter. Further, a meter generally records more gas than has actually flowed through it until the meter reaches a flow rate of about 60% or more of its maximum capacity. FIG. 1 shows a graph which is an example of the error curve for a turbine meter tested with air at atmospheric pressure.
Designers and manufacturers of turbine meters have used various changes in meter component structure and different methods to attempt to flatten the low flow-rate hump in the accuracy curve. By way of example, the hump in the accuracy curve of turbine meters at low flow-rates has been adjusted by adjusting rotor blade tip clearance with respect to the body of the housing in which the rotor is mounted. Generally, if the accuracy registered at low flow-rates is significantly lower than the accuracy at high flow-rates, the tip clearance is decreased to bring the low flow-rate end of the accuracy curve up toward the high flow-rate end. If the accuracy registered at low flow-rates is significantly higher than the accuracy registered at high flow-rates, the tip clearance is increased to bring the accuracy of low flow-rates down to the level of that at higher flow-rates.
Other attempts have been made to adjust the accuracy of liquid turbine flow meters by providing such things as a meter housing having a bore with a conical axial cross section adjacent the location where turbine blades are rotating on the meter's rotor assembly. When attempting to apply these principles of accuracy adjustment to turbine meters, it is desirable to have the angle of the conical axial cross section of a meter as large as practicable. This enables the accuracy of a turbine meter to be adjusted as desired with relatively little adjustment of the position of the turbine blades. It was found, however, that turbine meters having conical axial cross sections with angles such as about 21 degrees had their accuracies at higher flow-rates drop off to a range of from about 97% to 98%. This is generally an unacceptable range of accuracies for turbine meters.
One of the objectives in testing and calibrating turbine meters in the natural gas industry is to require that turbine meters be tested at a pressure commensurate with their intended use. The reason that turbine meters are tested at a pressure commensurate with their intended use is due to the known sensitivity of turbine meters to pressure. As a result, the calibration of a turbine meter at its intended operating pressure results in more accurate measurements. Currently, air and natural gas are widely used for the purpose of testing and calibrating turbine meters for the natural gas industry.
It is difficult to attain the objective of testing turbine meters at their use pressures and rotating speeds because to do so requires the construction of high-pressure test facilities, which are very expensive to construct and costly to operate. In addition, there are a number of criteria that need to be fulfilled in order to have a proper certified facility that can test gas meter turbines. The criteria require that test facilities be available with acceptable flow rates, test pressures, accuracy and traceability characteristics to perform meaningful, metrologically-sound tests.
There are currently several governments throughout the world that are looking more seriously at high-pressure test requirements and are considering guidelines setting forth specific parameters with regard to testing and calibration. There are, however, only a limited number of facilities worldwide that can meet the high pressure requirements for testing turbine meters.
As of September 2001, the European Community has adopted EN 12261, a new standard for the testing and calibration of turbine meters. It requires that turbine meters intended for use above four bars (gauge) be calibrated at a pressure not less than half, and not more than twice the operating pressure. Meters intended for use below four bars can be calibrated at atmospheric pressure. This new standard replaces a hodgepodge of national and International Organization of Legal Metrology standards that had no consistency regarding test pressures for turbine meters. The new standard reflects the consensus in Europe that turbine meters are sensitive to changes in the density of the gas they are measuring. Hence, to obtain a calibration that will be valid at operating conditions it is important to replicate the operating conditions at the time of testing. The European Community has recognized that atmospheric test conditions do not produce a satisfactory calibration for meters intended for use at medium or high pressure by implementing these new standards. Similar requirements are expected to be developed in the United States and Canada.
In the United States, there are currently no national regulations affecting the calibration of gas meters. Some States set requirements for domestic gas meters, however, few if any affect transmission or other high pressure metering. Instead, recommended practices are set out in voluntary reports published by the American Gas Association (AGA). Currently, the AGA report on turbine gas meters, AGA Report No. 7: The Measurement of Natural Gas and Turbine Meters (AGA-7), is under review. One of the major thrusts of the revisions to AGA-7 is to recognize the density sensitivity of turbine meters. The Transmission Measurement Committee of the AGA is continuing to study the topic and changes to AGA-7 are expected to result in far higher demand for high pressure calibrations in the United States to improve the accuracy of the turbine meters sold and used there.
Similarly, the regulatory requirements in Canada allow high-pressure testing but do not require it. Currently, calibrations of turbine meters at atmospheric pressure with air is the norm, although some meters are supplied by their manufacturers with optional high-pressure test and calibration data at additional cost. A draft specification was issued by Measurement Canada for industry comment in April 2001 that dealt with high pressure testing. This draft was based on a precursor of EN 12261 and included the idea of a dividing line between high- and low-pressure meters at four bars.
One of the concerns in Canada is that there are not enough high pressure test facilities available. Because of the high pressures and high rotation speeds involved these facilities are expensive to build and can be dangerous to operate.
Based on the proposed changes in the guidelines and regulations discussed above, there is likely to be a marked increase in demand for the calibration of high pressure turbine meters. The current facilities available would not be able to handle the anticipated new demand of testing requirements. To continue to build facilities similar to the ones already in existence would continue to increase the cost of testing turbine meters as the facilities are costly to build, expensive to use and have long turn-around times for service.
For the forgoing reasons there is a need to provide an improved method and system for proving turbine meters that results in a more efficient and accurate calibration of turbine meters.